Diseases of the periodontal tissues are increasingly regarded as disorders caused by bacteria which accumulate at the gingival margin and in the gingival crevice. A complex variety of gram-positive and gram-negative anaerobes and aerobes comprise this periodontal flora. There appears to be a procession of various bacterial types from the supragingival area to the base of a periodontal pocket. It is not clear which organisms are important pathogens and which are opportunistic commensals. Some will cause periodontal disease in experimental animals. A major obstacle to understanding the precise role of these bacteria is the inadequacy of their taxonomy. Many periodontal organisms cannot be identified; others are similar, but not quite identical to recognized species. We propose to clarify the taxonomic relationships of several periodontal bacteria by comparing the base sequences of their deoxyribonucleic acids (DNA). DNA will be extracted from cells lysed by combinations of penicillin, lysozyme, and sodium laurylsulfate. The molar ratio of guanine and cytosine in the total DNA bases will be determined by thermal denaturation. DNA base sequence similarities will be assessed by hybrid reassociation on membrane filters, and in solution through the use of single-strand specific nuclease(s). Organisms sharing 80% or more common sequences will be considered to be the same species. Below 50%, organisms will not be considered as members of the same species. Between 50 and 80%, other taxonomic criteria, such as phenotypic similarity and pathogenic similarity, will be considered in determining taxonomic position. We propose to study specific strains resembling Eikenella, Bacteroides, Capnocytophaga, and Fusobacterium.